by Patricia Sellers

One of the most interesting rising stars in Silicon Valley landed a CEO job this week. Sukhinder Singh Cassidy, the former President of Asia-Pacific and Latin American Operations at Google until last April, is the new CEO of Polyvore. The three-year-old company is an online platform to do “social shopping” for fashion and lifestyle products.

Social shopping? I had never heard of the concept, or of Polyvore, until Singh Cassidy called yesterday to tell me her news. Polyvore invites users to assemble and display collections of items–clothing, shoes, home goods, and more–to see how they look together. For a Polyvore user, the transaction component–buying opportunity on sites like Saks or Neiman Marcus or Net-a-Porter–is only a click away.

Polyvore co-founder Pasha Sadri, an engineer who came out of Yahoo and Google, says that the idea was to marry commerce and community, providing a better visual display than you can find on other websites. Some 6 million unique users visit Polyvore each month, he says, to mix and match items from any online store and create personal collections, or “sets” as the company calls then. Users create 30,000 new sets daily. Revenues, which he would not disclose, come from ads on the site–from the likes of Coach , Gap , Burberry and YSL–and commissions for directing traffic to other sites. The revenue opportunities are ripe for tapping and central to Singh Cassidy’s challenge as Polyvore’s new leader.

It’s not a surprise, actually, to see her arrive there. As Singh Cassidy was leaving Google to be a CEO-in-Residence at Accel Partners (which she was drawn to, she says, because Accel is prominent in the women’s e-commerce space), she got lured to join the board of  J. Crew by Mickey Drexler, its renowned merchant-CEO. A chic dresser by Silicon Valley standards, for sure (and more critically, by Manhattan’s measure), Singh Cassidy has been a Polyvore user for more than two years, ever since she met Sadri through Peter Fenton, a partner at rival VC firm Benchmark. Fenton is on the board of Polyvore and another hot little enterprise, Twitter. As she looks to accelerate Polyvore’s growth, Singh Cassidy predicts, “there will be opportunities for Accel and us to partner.”

An entrepreneur at heart (she co-founded an online financial-services company, Yodlee, in 2003), Singh Cassidy spent the past year scanning the universe for CEO jobs and decided on Polyvore a while ago. But she left Sadri and his team hanging until February 1. That was her 40th birthday. The ideal time to begin anew, she figures.

Later this week on Postcards: Singh Cassidy’s 12 Personal Career Guidelines. Check back to read wise advice from a player who is worth watching.